Liquid crystal displays are commonly used as display devices for compact electronic apparatuses. This is because they not only provide good quality images, but also they are very thin. Liquid crystal in a liquid crystal display does not emit any light itself. The liquid crystal requires a light source to display texts or images. Thus a typical liquid crystal display has a backlight module installed therewith.
Referring to FIG. 11, a typical backlight module 9 includes a light guide plate 92, a first diffusing film 93, a first brightness enhancement film (BEF) 94, a second BEF 95, and a second diffusing film 96 disposed in that order from bottom to top. Further, a light source 91 is provided at a side of the light guide plate 92.
Referring also to FIG. 12 and FIG. 13, the light guide plate 92 is essentially rectangular. The light guide plate 92 includes a light incident surface 921 adjacent to the light source 91, a top light emitting surface 923 perpendicularly adjoining the light incident surface 921, a bottom surface 922, and three side surfaces 924 perpendicularly adjoining both the light emitting surface 923 and the bottom surface 922. The bottom surface 922 defines a plurality of grooves 9222. The grooves 9222 are linear and parallel to each other. Each groove 9222 defines an essentially triangular profile.
Light beams emitted from the light source 91 enter the light guide plate 92 through the light incident surface 921. A majority of the light beams travel along generally perpendicular paths relative to the light incident surface 921 before reaching the grooves 9222. These light beams are reflected upward by interfaces of the grooves 9222, transmit out from the light emitting surface 923, and are utilized. A minority of the light beams travel along substantially oblique paths relative to the light incident surface 921 before reaching the grooves 9222. These light beams are reflected sideways by interfaces of the grooves 9222, transmit out from two lateral of the three side surfaces 924, and are not utilized. Accordingly, a light utilization ratio of the backlight module 9 is rather low.
Furthermore, the grooves 9222 all have a same configuration, and a pitch between every two adjacent grooves 9222 is constant. Thus the light beams emitting from the light guide plate 92 are not uniform. In order to output light uniformly, the backlight module 9 needs the two diffusing films 93, 96 and the two BEFs 94, 95. These plural films 93, 96, 94, 95 add to the cost of the backlight module 9, and make the backlight module 9 somewhat thick.
What is needed is an improved backlight module that can overcome the above-described deficiencies.